Both phones have their own static IP (192.168.16.201 &
192.168.16.202), and use the SBS server for DNS (192.168.16.1).
Gateway is 192.168.16.254 (Although for some reason when ever I make
a change to the configuration and reboot and logon the Gateway changes
to 192.168.255.0 ???)

The phones are on the same switch as the server (which has a single
NIC) and the PC's, which in turn is connected to the ADSL Modem.

5 Port Switch
Port 1=SBS, Port2=PC1, Port3=Spare, Port4= 8port Switch, Port5=ADSL
Modem

8 Port Switch
Port1 (POE)= Phone1 (192.168.16.201), Port2 (POE) = Phone 2
(192.168.16.202), Port7 = PC2, Port8= Unplink to 5 Port Switch. The
rest are spare.

It was sort of worked before I opened up the ports, this was done on
the advice of SipGate support FAQ.

It all seems a bit hit and miss at the moment.